In the case of a quadrature modulator of the aforesaid kind errors in amplitude and errors in phase can occur, for instance, in the high-frequency signal generating means, in the mixers and in the adder. Phase errors may also occur as a result of different wave lengths on, for instance, pattern cards in the modulator. In order to achieve the result desired, the amplitude error and phase error in the modulator should not exceed about 1/2 dB and about 3 degress respectively. It is difficult, however, to achieve such low values for frequencies which are higher than some 100 MHz. For example, the wave length at 1000 MHz is about 3 dm, which means that one (1) degree will correspond to a distance in the order of one (1) mm.
Local oscillator leakages occur as result of the leakage of signals through the mixers from the high frequency signal generating means. Leakage signals can result in so-called widening of the spectrum range, particularly when using non-linear transmission terminal stages, which in turn can result in disturbances in neighbouring traffic channels. It is known to attempt to counteract the occurence of amplitude and phase errors and local oscillator leakages, by using components which have relatively good properties, although costs then become comparatively high.